Light can be described as an electromagnetic wave that consists of periodically-varying electric “E” and magnetic “M” fields oriented at right angles to each other. Light propagates in certain intensity patterns, which are referred to as “modes.” One such mode is the “transverse” mode. Transverse modes occur because of boundary conditions imposed on a light wave by a waveguide. Transverse modes are classified into different types:                TE modes (Transverse Electric) have no electric field in the direction of propagation;        TM modes (Transverse Magnetic) have no magnetic field in the direction of propagation;        TEM modes (Transverse Electromagnetic) have no electric nor magnetic field in the direction of propagation; and        Hybrid modes are those which have both electric and magnetic field components in the direction of propagation.Light traveling in an optical fiber or other dielectric waveguide forms hybrid-type modes.        
Waveguides are never perfectly symmetric, nor are the forces that affect the waveguide applied in perfect symmetry around it. Any asymmetry in a waveguide, especially a single-mode waveguide, can cause differences in the manner in which polarization modes propagate through the waveguide. More particularly, due to waveguide asymmetries, the TE and TM polarization modes might experience somewhat different propagation conditions such that they: (1) travel along the waveguide at slightly different speeds and (2) experience differing amounts of attenuation. The former effect, which results in “polarization mode dispersion,” causes a spreading of an optical pulse that can render it unreadable at a detector. The latter effect, known as “polarization-dependent loss,” results in signal losses. Both of these problems can degrade the operation of high-performance, high-speed optical systems. For such applications, it is therefore desirable to produce optical components and systems that are effectively insensitive to polarization mode, a quality referred to herein as “polarization independence.”
Polarization independence is discussed further below in the context of ring resonators.
Ring Resonators. An integrated ring resonator is a potentially very important optical device primarily for use in optical communications systems. The integrated ring resonator is seen as a promising replacement for Mach-Zehnder interferometer (“MZI”)-based devices, among other devices, which are used to form optical “components” such as filters, routers, switches, and the like. The integrated ring resonator consists of a closed-loop waveguide and one, or more typically two, linear waveguides. Certain frequencies of light resonate within the loop waveguide and, as a consequence of resonator geometry, resonant light couples between the loop waveguide and the linear waveguides. The ring resonator therefore effectively functions as a highly-selective wavelength-dependent optical coupler. A device that is functionally identical to the ring resonator is the disk resonator. The difference is that rather than incorporating a ring or loop waveguide, the disk resonator includes a solid disk waveguide.
In most implementations, light energy is coupled into and out of the micro-resonator's loop waveguide via evanescent field coupling. An evanescent optical field is the portion of the optical field of guided light that extends beyond the physical surface of a waveguide. In this coupling mode, the loop waveguide is placed in close proximity to both of the linear waveguides. All of the light at the resonant wavelength is eventually transferred from one linear waveguide to the other linear waveguide. Light within a linear waveguide that is off-resonance (i.e., not at the resonance wavelength) bypasses the loop with only a small transmission loss.
Prior-Art Resonator 100. FIG. 1A depicts a typical integrated ring resonator 100, which consists of two linear waveguides 102 and 104, and a third waveguide 106 in the form of a closed loop (hereinafter “loop waveguide 106”). The loop is typically circular, oval or elliptical, but can alternatively have an arbitrarily curved circumference in the form of a distorted ring. The linear waveguides are usually termed “port waveguides” or “waveguide buses;” the former phrase will be used herein.
Integrated ring resonators are fabricated in two geometries: “laterally coupled” and “vertically-coupled.” Resonator 100 of FIG. 1A is an example of a laterally-coupled resonator. The designation “laterally coupled” refers to the fact that port waveguides 102 and 104 are in the plane of loop waveguide 106. In a vertically-coupled resonator, the port waveguides 102 and 104 are situated either both above, both below, or one above and one below loop 106 (see, FIG. 1B). Each of these two basic geometries has well-understood advantages and drawbacks.
Structure of Resonator 100. FIG. 2 depicts a schematic representation of prior-art resonator 100. As depicted in FIG. 2, resonator 100 has four ports: input port 202A and pass port 202B defined on port waveguide 102 and add port 204A and drop port 204B defined on port waveguide 104.
A portion of each of port waveguides 102 and 104 is adjacent and tangential (i.e., in the direction of a tangent) to loop waveguide 106. At this portion of the port waveguides, they are separated from loop waveguide 106 by gap G. In some embodiments, this gap is filled with air or one or more materials having a refractive index that is higher than air.
In some alternative implementations (not depicted), respective portions of each of waveguides 102 and 104 that are adjacent to loop waveguide 106 are not tangential thereto; rather, they curve around a portion of loop waveguide 106 to increase the interaction length (for coupling). In some further implementations, a vertically-coupled ring resonator includes port waveguides that are disposed orthogonally to one another (see, e.g., U.S. Pat. No. 6,411,752).
Operation of Resonator 100. Certain wavelengths of light resonate within loop waveguide 106 as a function of loop characteristics. Consider a multi-wavelength optical signal propagating in port waveguide 102 past input port 202A. Light that has a wavelength that is off-resonance with loop waveguide 106 bypasses the loop and is output from pass port 202B of port waveguide 102. Light that has a wavelength that is on-resonance couples to loop waveguide 106 via evanescent field coupling.
The on-resonance light that is coupled from port waveguide 102 propagates in loop waveguide 106 and couples to port waveguide 104 via evanescent field coupling. The light that is coupled into port waveguide 104 propagates in a direction opposite to the light traveling in port waveguide 102 due to the respective orientations of the various waveguides. As a consequence, the resonant light coupled to port waveguide 104 from port waveguide 102 via loop waveguide 106 will be output from drop port 204B. This resonant light will be joined by off-resonance light that propagates along waveguide 104 from add port 204A.
In analogous fashion, on-resonance light traveling in port waveguide 104 via add-port 204A couples to loop waveguide 106. That light couples to port waveguide 102 and propagates through pass-port 202B, along with off-resonant light from input port 202A. Resonator 100 is operated so that light propagates unidirectionally—in this example in a counterclockwise direction—through loop waveguide 106.
Design Considerations for Resonator 100 It is desirable for resonator 100 to be very small so that: (i) its free spectral range is large and (ii) it occupies very little physical space, thereby enabling large-scale integration. In fact, a typical ring resonator has a diameter that is only one or two orders of magnitude greater than its operating wavelength. So, for a telecommunications application having an operating wavelength of about 1.55 microns, loop diameter is usually in the range of about 15 to about 150 microns.
Due to their small size, optical ring resonators are usually called “micro-resonators.” That term will be used henceforth to refer to any of the many implementations of optical ring or disk resonators (e.g., circular, oval, elliptical, distorted versions thereof, etc.).
To guide light around the tight radius of curvature of loop waveguide 106, the dielectric contrast (i.e., difference in refractive indices) between the loop waveguide and surrounding medium in the plane must be large. Micro-resonators, including integrated port waveguides, are usually fabricated in substrates such as Si/SiO2, GaAs/AlGaAs, and Si3N4/SiO2 to facilitate their incorporation into optical systems. For this reason, the micro-resonators are often referred to as “integrated” micro-resonators.
In order to function properly, the effective refractive indices of loop waveguide 106 and port waveguides 102 and 104 should be as close in magnitude as possible. Another design consideration is that most applications for integrated micro-resonators require that port waveguides 102 and 104 are single mode waveguides.
In most applications, it is very important for the micro-resonator and components that incorporate it to be as insensitive as practical to the polarization state of the transverse polarization modes, the goal being polarization independence.
The Difficulty of Achieving Polarization Independence. It is quite challenging to produce a polarization-independent, integrated micro-resonator. This is due to the geometry of the resonator, which exhibits bending in the horizontal plane. This causes the TE polarization mode to experience a higher refractive index and lower losses in loop waveguide 106 than the TM polarization mode. Although it is theoretically possible to alter ring width and height to equalize the treatment of the polarization modes, a number of vexing problems arise when attempting to do so.
One problem with altering ring width and height is that not all materials can be used to fabricate the ring in the dimensions that are required for polarization independence due to factors such as material stress, refractive-index requirements, and processing constraints. A second problem is that even if a material possesses the requisite material parameters, there remains the issue of matching the refractive indices of loop waveguide 106 and port waveguides 102 and 104 while maintaining mono-modality and polarization independence of the port waveguides.
A third problem pertains to the issue of fabrication tolerances. That is, in order to produce an integrated micro-resonator with a desirably-small diameter, the resonator must be fabricated in materials that have a high dielectric contrast, as previously mentioned. This, on the basis of maintaining mono-modality, implies relatively smaller waveguides. But smaller waveguides are far less tolerant than larger waveguides to variations in the fabrication process. As a consequence, the polarization-independent behavior of integrated micro-resonators is very difficult to guarantee since even a small variation in either the width or height of loop waveguide 106 will cause polarization dependence.
Polarization Diversity as an Alternative to Polarization Independence. As will be clear from the foregoing discussion, it is difficult if not impossible to reliably manufacture polarization-independent integrated micro-resonators. In recognition of this difficulty, an alternative to controlling waveguide geometry/materials was developed in the pursuit of polarization independence. And that is to treat each polarization mode independently in separate, discrete integrated micro-resonators, thereby creating a polarization diverse arrangement of micro-resonators.
FIGS. 3A and 3B provide illustrations of a wavelength filter implemented via micro-resonators to illustrate the distinction between polarization independence and polarization diversity.
In FIG. 3A, the filter is implemented using a single micro-resonator 300 having port waveguides 302 and 304 and loop waveguide 306. On-resonance wavelengths of an input optical signal propagating along port waveguide 302 couple to loop waveguide 306 and then to port waveguide 304 for drop. Off-resonance wavelengths pass the loop waveguide. Since the traverse modes (TE and TM) are not separated, the micro-resonator must exhibit polarization independence. In this regard, the resonator depicted in FIG. 3A is an idealized layout, since polarization independence cannot be assured.
FIG. 3B depicts a polarization-diverse implementation of a filter equivalent to the filter of FIG. 3A. The filter depicted in FIG. 3B includes polarization splitter 310, two micro-resonators 300A and 300B, three polarization rotators 312, 314A, and 314B, and two polarization combiners 316A and 316B.
Those skilled in the art are familiar with the use of polarization splitters, polarization rotators, and polarization combiners and, as such, only a brief description of these components will be provided.
A polarization splitter is used to separate the polarization components TE and TM into separate signals. The operation should be wavelength-independent and the resultant signals should have the same wavelengths. Polarization splitters have been implemented via passive or active directional couplers, asymmetric Y-junction couplers, multimode interference couplers, 2-d grating couplers, and photonic crystals, to name a few.
A polarization combiner is simply a polarization splitter that is run in reverse. In other words, a TE and a TM signal having the same wavelength(s) are combined into a single signal.
A polarization rotator maneuvers linearly-polarized light about an optical axis, and can change the polarization state of a signal from TE to TM or vice-versa. Examples of polarization rotators include, without limitation, optical rotators, Faraday rotators, and half-wave plates.
With continued reference to FIG. 3B, at the input of the filter, polarization splitter 310 is used to separate multi-wavelength optical signal A, which exhibits both TE and TM mode polarizations, into two signals B and E each having different transverse polarizations. Signal B, which has TE polarization, propagates along port waveguide 302A. Signal E, which has TM polarization, propagates along port waveguide 302B. The polarization of signal E is immediately converted to TE in polarization rotator 312.
After polarization conversion of signal E, both signals B and E have the same polarization (TE in this case). On-resonance light from signal B couples into loop waveguide 306A and then to port waveguide 304A as signal D heading toward polarization combiner 316B for drop. Similarly, on-resonance light from signal E couples into loop waveguide 306B and then to port waveguide 304B as signal G heading toward polarization rotator 314B. The polarization of signal G is then converted back to TM in polarization rotator 314B. Signal G having TM polarization is then combined with signal D having TE polarization in polarization combiner 316B to form signal H having TE and TM polarizations. The filter drops signal H.
In summary, the following operations are conducted in the polarization-diverse arrangement described above:                (i) splitting the original signal A into two signals with different polarizations;        (ii) changing the polarization of one of those signals so that the two signals have the same polarization;        (iii) providing the two signals to duplicate parts of the filter;        (iv) extracting on-resonance wavelengths from the two signals via two wavelength-dependent optical couplers, creating two new signals;        (v) changing the polarization of one of the new signals so that the two new signals have different polarizations; and        (vi) recombining the two signals.        
Operation (ii), which involves changing the polarization of one of the signals so that both signals have the same polarization, ensures that the two signals B and E (and signals based thereon) will respond identically to prevailing travel conditions. It also enables the system to be optimized for a particular mode; in this case, the TE mode. Operation (iii) ensures that the two signals (i.e., B and E) will experience identical travel conditions.
Regarding off-resonance signals F and C, the polarization state of signal C is converted to TM in polarization rotator 314A and led to polarization combiner 316A. Signal F, which has TE polarization, is also led to polarization combiner 316A. Signals C and F are then combined in polarization combiner 316A to form “pass” signal I, which exhibits both TE and TM polarizations.
The polarization-diverse scheme depicted in FIG. 3B solves many of the aforementioned design and fabrication problems that otherwise arise when designing a polarization-independent filter. Unfortunately, this solution introduces a new set of problems.
The Drawbacks of Existing Polarization-Diverse Layouts. One problem introduced by the polarization-diverse scheme depicted in FIG. 3B is that additional chip area is required to implement the filter due to its enlarged footprint (compared to the filter of FIG. 3A). But more problematic are the difficulties that pertain to the control of the resonance frequency of the complementary pair(s) of micro-resonators in the filter.
In particular, it is one thing to design a micro-resonator to operate at a particular resonance frequency, but it is quite another for a fabricated micro-resonator to actually operate (without modification) at the design resonance frequency. In fact, micro-resonators usually require some form of “tuning” to operate at a design resonance frequency. Regardless of any deviation from design parameters, tuning is often required to implement some desired functionality (e.g., switching, modulation, etc.). Most micro-resonator designs therefore employ some type of active tuning, such as electro-optic or thermo-optic, to adjust resonance frequency.
Since tuning requires an electronic driver for each individual micro-resonator, and each driver and the tuning method have an associated power dissipation, the power dissipation is approximately linearly dependent on the number of micro-resonators. The power requirement for a polarization diverse implementation is therefore approximately twice that of an implementation that uses polarization-independent micro-resonators, were such a polarization-independent arrangement possible.
Furthermore, since each micro-resonator must be connected to a driver, the pin count of the polarization-diverse implementation is greater than would otherwise be the case. Also, the polarization-diverse scheme requires the resonance frequencies of the complementary micro-resonators to be the same in order to avoid signal degradation.
These strict requirements increase the complexity of the driver electronics and add to the number of device parameters the must be validated during manufacturing of prior-art polarization-diverse optical systems. All of the foregoing issues increase the costs of a polarization-diverse implementation relative to a polarization-independent implementation of an optical filter.
A need exists, therefore, for optical components that are effectively insensitive to polarization (either via true polarization-independence or via polarization-diversity) but that avoid at least some of the drawbacks of the prior-art.